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You can certainly use a filter wheel and focus each colour separately. Nothing wrong with that idea at all. If you are using filters, you might want to consider using narrowband filters (the usual trio are Hydrogen Alpha, Oxygen 3 and Sulphur 2) with the narrowest bandpass (width of the spectrum it will allow through). A bandpass of 5nanometers is fine although there are some available with even less). That way, even in the burbs, you can image deep space objects without having to be too bothered by street lights andn such.
Having said that, you might also want to think twice about buying a cheap achromat instead of a triplet. It is probably a false economy and you will quickly become frustrated by its limitations.
The things that will turn your imaging from ho-hum to whacko will be your guiding though. The mount will track reasonably well but for imaging you need to help the mount along by using a guide camera as well. These are either attached to a separate telescope mounted on top or beside the main scope, or they are built into an off-axis guider that does away with the weight, cost etc of a second scope.
My best suggestion is buy as good a triplet as you can now afford (they hold their resalem value well) and an off-axis guider and guide camera such as a QHY5 or similar.
Then you have to familiarise yorself with them and with the methods of aligning your telescope to the celestial south pole.
It's a learning curve but a rewarding one. And there will always be a heap of advice available in these fora.
Peter
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